So everyone outside the know makes the odd assumption that cyclists wear tight clothes "spandex" because it is aerodynamic.
It's true that wearing "spandex" from neck to knee is somewhat aerodynamic, but there are other major considerations going on here.
In the broad scale, the human body is very much not aerodynamic. Aerodynamics are important when cycling at high speed, above 20mph, but bike races are not won at high speeds. When cyclists are racing at high speeds, there's ususally a large pack of riders who ride in other rider's slipstream. They're usually won on long, steep hills....where even the fastest guy might be riding at 12mph. Not so much for aerodynamic considerations at that speed.
If being aerodyamic was the primary goal of such clothing, we'd also all wearing an aero helmet and be on a time trial bike like Dave Zabriskie here.
So why "spandex" then? A few things:
1. You, the casual observer, aren't able to see huge sweatspots. You should actually be applauding the fact that we aren't subjecting you to huge splotches of wetness emanating from our underams, chest, and crotch. We'd probably get way more crap if we wore cotton.
2. It doesn't chafe. Ever go to the gym wearing streetclothes and work out *hard* for an hour? If you haven't, I'll fill you in. Anywhere your body has been moving, the wet clothes will have abraded away your skin and chafed. It even happens with the best of clothes.
3. It doesn't stay wet. Generally, cyclists are rarely on a ride for less than an hour. On just a warm day, you can sweat off a few liters of fluids in an hour. Wet clothes are unconfortable and unsightly (see #1 & #2)
Same goes for helmets. the sculpted helmets you see cyclists wearing are also not really aerodynamic. They're designed keep your head from overheating on a 95F day when you're not cracking your skull to the pavement after being buzzed by an oblivious soccer mom on her cell phone.
Also, people don't wear cycling clothes to look like Lance Armstrong. If anything, pro cyclists are sponsored by performance clothing companies in the same way Tiger Woods & Kobe Bryant sell sporting apparel. When I see a neighborhood game of b-ball, and see some guys wearing jerseys, I don't think, "what a moron! that guy thinks he's Shaq!"
Perspective: Drivers of horse carriages were constantly annoyed by bicyclists at the turn of the century 20th in New York City; little has changed in over 100 years. And should the day come that we're rationing our dwindling petroleum reserves for strict consumption for international air transport and agricultural equipment, I'm sure whatever you're using to get around, a cyclist will find some way to piss you off.